The inherent shrinkage experienced during firing in typical prior art ceramics can be as much as thirty percent. U.S. Pat. No. 5,102,720 reports that:
"The state-of-the-art in multilayer ceramic substrate technology has been represented by the IBM multilayer ceramic substrate as described in Blodgett, A. J. et al., IBM J. Res. Develop. 26,1, pp 30-36 (1982). . . . (That) substrate shrinks in the x, y and z directions during sintering, approximately 17% in each direction, and such shrinkage in the x-y plane may cause interface defects and delamination."
Conventional ceramic products are generally formed by the "green tape", "dry press" or "extrusion" process from pastes, powders or slurries of alumina or other ceramic materials mixed together with glass powders and organic solvents or solutes. The organics are volatilized at substantially lower temperatures than the firing or processing temperature of the ceramic bodies or substrates. Solvents usually evaporate at temperatures below about 100.degree. C. and solutes evaporate at temperatures below about 450.degree. C. The loss of the solute and solvent leaves pores in the green tape or cold pressed body. At the peak firing temperatures the glassy phase melts, a certain amount of sintering of the alumina particles occurs and there is a resulting filling of the voids or densification of the bodies. It is this densification which is a factor in shrinkage.
Investigation has also shown that shrinkage can be the result of mechanically and chemically combined water being driven off, evolution of gasses due to dissociation of raw materials, and formation of glasses and/or crystalline phases of higher specific gravity. The degree of change can also be affected by chemical composition, particle size, particle size distribution and particle shape of the raw materials, firing profile, ignition losses, and forming methods. It has recently been determined that the size distribution of the glass particles plays an important part in shrinkage and thus shrinkage control.
Equally as troublesome to workers in the field as the absolute amount of shrinkage is the extreme difficulty involved in predicting with any degree of certainty, what the actual percentage of shrinkage will be for a given lot or piece. For example, average shrinkage for a given composition and firing profile may be a certain percentage. However, it is just that, an average, and the actual shrinkage from lot to lot and piece to piece may vary by as much as plus or minus one or two or three percent or more from that average. Clearly, this is unacceptable when the design tolerances between two parts require a greater degree of precision.
This shrinkage and lack of predictability presents particular difficulties in the electronics industry where layers of "green" ceramic containing pre-set horizontal and vertical conductive pathways or vias, are stacked together and then fired. If the conductive pathways do not match up in the fired state as they did in the green state because of differing shrinkage rates for one or more of the layers, the resultant part either has to be reworked or scrapped; all of which adds unnecessary expense and time to the manufacturing process.
There have been several efforts to control or eliminate shrinkage by, e.g. pre-igniting or calcining the raw materials, reducing the binder and thus the void-causing volatile content, increasing forming pressures, controlling particle morphology, mechanically constraining the part to restrict shrinkage to the vertical plane, and by formulating with materials that form crystal phases which are larger than their precursors. Each approach has had varying degrees of success. However, no one has been able to consistently reduce firing-induced shrinkage to less than about two percent and preferably less than about one percent.
There thus exists the need for ceramic compositions with minimal shrinkage, i.e., less than two percent and preferably less than one percent, during the firing process. Equally as important there exists the need for ceramic compositions where the shrinkage from lot to lot and piece to piece is both predictable and substantially the same.